cutting and patching a lower square in my basement concrete slab for treadmill

So I have a finshied basement (carpet with underpadding on concrete and drywalled ceiling) that I want to put a new treadmill in. With the height of the raised treadmill and myself running on it, it will be just and too uncomfortable to run. So I have decided to cut a rectangular area in my concrete slab floor to allow me to lower the treadmill into. From reading many posts I have decided to rent a 12inch electric Hilti concrete saw with attached vacuum from my local Home Depot to make clean cuts and then slegdehammer it into manageable pieces to remove. Then I will have to dig 17inches to allow for my 10inch lowered finished trough, 4inches of concrete slab and 2-3inch rock backfill. My big question is how to frame to pour the concrete? Can it be done in one shot or do I have to pour the lowered trough flat bottom first and then once it's dried, make a rectangular frame to create my perimeter walls between the existing concrete slab floor and the new lowered trough walls? I have read advise that I will need to rough up the clean cut lines for the new concrete to adhere well to the existing (cold chisel and hammer or grind channels?) Is drilling holes in the side of the existing slab to insert rebar dowels to connect the old and new concrete a must in this situation, the treadmill weights a couple of hundred pounds plus me on it?

Any advice would be greatly appreciated, especially if I am off track with my thinking.

Any idea where the water table is? Is there insulation and a plastic sheet underneath the slab? What about drainage pipe? While it is unlikely you have a post-tensioned slab, if you do, cutting it can be quite dangerous. To minimize cracking, you will need rebar, and yes, drilling into the existing slab then probably epoxying the pieces in is likely the best way (get some expert advice on that to be sure).

If I ever move and build a house, I think I'd do what a friend did - he has 10' ceilings in his basement! He uses it to store books, as he's a used book dealer, and the height is great. High enough to do almost anything from a big theater, etc.

For just the cost of making that hole, you could buy yourself a top quality pair of jogging or running shoes and run outdoors where you don't get bored of running, and you've got more than enough head room.

You have several options. You can make a monolithic pour doing the whole thing at once. OR, pour the floor and then the walls. Or, pour the floor and use concrete blocks for the walls. These are just three options and there are others also..

For just the cost of making that hole, you could buy yourself a top quality pair of jogging or running shoes and run outdoors where you don't get bored of running, and you've got more than enough head room.

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Well I do run outdoors spring, summer and fall but not an option for me come winter time. I am considering just putting it in my bedroom. I hadn't considered the water table and I am not 100% sure of the drain pipe location either. Wouldn't want to open a whole can of worms...

You have several options. You can make a monolithic pour doing the whole thing at once. OR, pour the floor and then the walls. Or, pour the floor and use concrete blocks for the walls. These are just three options and there are others also..

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Just for my knowledge, what would the form look like for the single pour? Something like a stair form?

Run the treadmill parallel to main level floor joists then cut out and box in one bay which should give you a minimum of 7 1/4" extra headroom (if empty, 2x8 construction). This would hurt the resale value less than a damp pit a potential buyer has to fill in.